1. Field of the Invention
This invention pertains to a method and apparatus for logging the gravel pack zone of an oil/gas well immediately after it has been gravel packed. The apparatus is attached to the gravel pack service tool prior to commencing the gravel pack installation process. After completion of the gravel pack installation process, the gravel pack density is logged while the gravel pack service tool is being withdrawn from the drill hole.
2. Description of the Prior Art
Production of formation sand is a frequent problem of oil/gas wells. When sand production is significant, it may fill the well, plug the perforations in the production screen, fill the gas/oil separators and in some cases cut through the pipes and valves. In some cases sand production may weaken the formation, leading to formation and casing collapse and loss of the well. The costs of remedial action are in many cases enormous.
The principal methods of controlling sand production include limiting the production rates, consolidating the formation by chemical means and installing a gravel pack between the formation and the screening device in the well bore. With the gravel pack method, the gravel pack acts as a filter and holds the formation sand out of the well bore while permitting maximum gas or oil production. Such a gravel pack method and apparatus is disclosed in U.S. Pat. No. 4,044,832, issued Aug. 30, 1977 to Richard, et al. for "Concentric Gravel Pack With Crossover Tool and Method of Gravel Packing."
Voids may occur in the gravel pack during its installation. These voids usually are not evident during the installation process, but rather, become evident only after the gravel pack installation process is completed, and sometimes only after the well is in production. Voids in the gravel pack permit the flow of sand from the formation into the well bore during production. These may lead to early failure of the well requiring expensive remedial workover. Therefore, it is desirable to evaluate the installed gravel pack and to detect any voids soon after installation and before production.
A method of logging the density of the material surrounding well holes with a radioactive source and a detector is described in U.S. Pat. No. 4,492,865, issued Jan. 8, 1985, to Murphy, et al. for "Borehole Influx Detector and Method". This method is adaptable for logging gravel packs. The radioactive source emits gamma radiation, which impacts the material surrounding the well bore. The effects of the emissions impacting the volume of material between the logging tool and the formation are measured by the detector. By suitable choice of source and detector, the logging tool is made responsive to the density of the material in the volume surrounding the tool. Because the well bore fluids in general have less density than the materials used in a gravel pack, a void in the gravel pack is revealed by an area of reduced density.
Current practice is to remove the gravel pack service tool equipment from the well bore after completion of the gravel pack installation. A density logging tool is then lowered on an electric wireline and a density log is run of the interval of the well containing the gravel pack. See, for example, U.S. Pat. No. 2,156,052, issued Apr. 25, 1939 to Cooper for "Logging Device." Because different equipment is used for the installation of the gravel pack and for logging the gravel pack density, there is a delay between the gravel pack installation and the subsequent logging while the gravel pack equipment is removed and the logging tool is lowered into position. The delay between the gravel pack installation and logging run means additional rig time and service company costs. Moreover, if a void in the gravel pack installation is indicated by the data obtained from the logging run, then the gravel pack service tool equipment must be re-inserted into the well bore for remedial action. In some cases, the gravel pack service tool equipment may have been removed from the well site for use at another well site, thereby resulting in additional delays and costs before the void is corrected.
It is desirable to reduce such costs and delays by detecting any voids immediately after completion of the gravel pack installation. While methods for downhole logging are known in the art (see U.S. Pat. No. 4,529,939, issued July 16, 1985 to Kuckes for "System Located in Drill String for Well Logging While Drilling"; U.S. Pat. No. 4,492,865, issued Jan. 8, 1985 to Murphy, et al for "Borehole Influx Detector and Method"; and U.S. Pat. No. 4,041,780, issued Aug. 16, 1977 to Angehrn for "Method and Apparatus for Logging Earth Boreholes"), these methods, unlike the present invention, do not disclose the detection of gravel pack voids immediately following gravel pack installation as the gravel pack service tool is withdrawn. The present invention provides such capability.
In brief, the density logging tool of the present invention is connected to the lower end of the gravel pack service tool equipment before it is lowered into the well bore for gravel pack installation. The logging tool, which in the preferred embodiment does not require any electric wirelines, remains inactive until the gravel pack installation is completed. As the gravel pack service tool equipment begins to be removed from the gravel pack assembly and well bore following completion of the gravel pack installation, the density logging tool is automatically activated and the gravel pack density is logged as the logging tool is withdrawn along with the gravel pack tool. The logged density data is stored digitally in the logging tool. The present invention also includes capability to limit significant power consumption to the periods when gravel pack density data is taken.
The data collected by the density logging tool is transferred from the logging tool to a surface module after the logging tool is retrieved at the surface. Then, in the preferred embodiment, the digital data is stored for archival purposes on a cassette recorder in the surface module, and a strip chart recording is made of the density log data for immediate analysis. If the analysis of the log indicates that voids are present in the gravel pack, then remedial action can be initiated immediately. The principles of operation of such a tool and the preferred embodiment are described below. In an alternative embodiment, the digital data can be transmitted by appropriate telemetry or other transmission means to the surface for evaluation in real time, or after a time delay, but before the density logging tool is retrieved at the surface.